D. Armentrout

Failure analyses of nonceramic insulators. Part 1: Brittle fracture characteristics

Nonceramic insulators, also referred to as composite, polymer or polymeric insulators, are used in overhead transmission lines with line voltages in the range of 69 to 735 kV. Despite the many benefits that nonceramic insulators offer in comparison with their porcelain counterparts, they can fail mechanically in service by rod fracture. One of the mechanical failure modes of the insulators is a failure process called brittle fracture, which is caused by the stress corrosion cracking (SCC) of the GRP rods. The process is catastrophic and unpredictable, leading to the drop of energized transmission lines. The most important characteristics of the brittle fracture process, which can occasionally affect high voltage nonceramic transmission line insulators, leading to their catastrophic in-service failures, have been presented in this article. In addition, several experimental techniques were suggested for the simulation of brittle fracture under laboratory conditions. Only the most important aspects of brittle fracture process have been discussed here.

Causes and potential remedies of brittle fracture failure of composite (nonceramic) insulators

Existing brittle fracture models have been reviewed and their applicability to explain the in-service brittle fracture failure of composite (nonceramic) insulators is evaluated. It is shown that the only brittle fracture model that can explain all aspects of the brittle fracture process is a model based on the formation of nitric acid solutions in-service. The chemical cause of brittle fracture is identified in this work and recommendations are made on how to avoid brittle fracture in-service by proper selection of composite insulator rods resistant to brittle fracture. An attempt is made to clarify misconceptions that exist in the literature regarding the causes of brittle fracture and the most suitable prevention methods.

Failure analyses of nonceramic insulators: part II - the brittle fracture model and failure prevention

In this work, an improved version of a brittle fracture model, based on the formation of nitric acid in service through corona discharges, ozone, and moisture, is presented and is used to explain several different modes of brittle fracture. Similar to Part I, we refer throughout this article to the insulators as nonceramic insulators (NCIs). To prevent brittle fracture in-service, its causes must be first established. To prevent brittle fracture in-service, its causes must be first established.

Boron-free fibers for prevention of acid induced brittle fracture of composite insulator GRP rods

An investigation was performed to determine whether corrosion resistant boron-free E-glass fibers could adequately prevent acid induced brittle fracture failures of high voltage composite insulator rods. Nine different rod compositions were tested at 45% of mechanical failure load in contact with 1 N nitric acid. Rods made out of commonly used E-glass fibers failed mechanically in less than 2 h whereas the rods based on the corrosion resistant boron-free fibers from two different suppliers survived four days of testing with no visible damage to the rods. Differences in resin types had little effect on the times to failure of the rods. Acoustic emission location analysis was also used to determine the location of fiber fractures along the rods. The location analysis revealed significant differences between the rods with the two different types of corrosion resistant fibers. Boron-free fibers with a lower seed (void) concentration exhibited noticeably fewer fiber fractures as measured by acoustic emission in comparison with the high seed boron-free fibers.

Evaluation of stress corrosion properties of pultruded glass fiber/polymer composite materials

A series of stress corrosion experiments were performed on 12 pultruded glass fiber composite materials (two fibers in six different matrices) commonly used in high voltage composite insulators. In this study two types of glass fibers were investigated, namely E-glass and ECR-glass. The tests were performed in nitric acid solutions (pH 1.2) using constant KI specimens that were specifically designed for the stress corrosion testing of unidirectional fiber/polymer matrix composites. The effect that the magnitude of the applied load had on the stress corrosion fracture process was investigated using acoustic emission (AE) methods. Post-test analysis of the test samples revealed that when the specimens were subjected to static loads ranging from 71.2 to 124.6 N in the presence of nitric acid, planar cracks formed and propagated perpendicular to the fiber direction without generating a significant degree of fiber debonding or pullout. The results showed that the E-glass/polymer unidirectional composites were not immune to stress corrosion cracking in nitric acid. On the other hand, the ECR-glass/polymer composites showed no evidence of stress corrosion formation under any of the loading conditions considered. Furthermore, the test results clearly demonstrate that the constant KI stress corrosion tests are a suitable experimental method for evaluating stress corrosion properties of unidirectional glass fiber/polymer matrix composites. In particular, by using the constant KI specimen geometry, the crack propagation rates in the composites under the stress corrosion conditions can be very accurately determined.

Resistance to brittle fracture of glass reinforced polymer composites used in composite (nonceramic) insulators

In this paper, the most important results are presented and discussed from a multiyear interdisciplinary study directed toward the identification of the most suitable glass/polymer composite systems with the highest resistance to brittle fracture for high voltage composite insulator applications. Several unidirectional glass/polymer composite systems, commonly used in composite insulators, based either on E-glass or ECR-glass fibers embedded in either polyester, epoxy, or vinyl ester resins have been investigated for their resistance to stress corrosion cracking in nitric acid. The most important factors (fiber and resin types, surface fiber exposure, polymer fracture toughness, moisture absorption, interfacial properties, sandblasting) affecting the resistance of the composites to brittle fracture have been identified and thoroughly analyzed. It has been shown that the brittle fracture process of composite (nonceramic) insulators can be successfully eliminated, or at least dramatically reduced, by the proper chemical optimization of composite rod materials for their resistance to stress corrosion cracking.

Water diffusion into and electrical testing of composite insulator GRP rods

This paper describes water diffusion into and electrical testing of unidirectional glass reinforced polymer (GRP) composite rods used as load bearing components in high voltage composite (non-ceramic) insulators. The tests were performed following ANSI standard C29.11 Section 7.4.2 that can be used to evaluate electrical properties of composites. The unidirectional composite rod materials based on either E-glass or ECR-glass fibers with modified polyester, epoxy and vinyl ester resins were investigated. Two types of ECR-glass fibers were considered, namely high and low seed (voids). The effects of composite surface sandblasting, mechanical pre-loading and nitric acid exposure on the electrical properties of the composites were studied. In addition to the required data of the ANSI standard, the specimen mass gain was also measured after boiling for 100 h. Most importantly, there was no correlation found between the mass gain and the leakage current for different composites. The materials with high seed ECR-glass fibers had much higher leakage currents and they absorbed less moisture than the composites based on either the low seed ECR-glass fibers or E-glass fibers. It was shown in this work that different types of sandblasting, as well as mechanical preloading with and without acid exposure had a negligible effect on the leakage currents and water mass gain of the composite specimens.

Failure prediction analysis of an ACCC conductor subjected to thermal and mechanical stresses

In this work, the Aluminum Conductor Composite Core¿ (ACCC) was numerically investigated to evaluate stress distributions when subjected to thermal and mechanical loads. The thermal analysis was conducted to simulate the cooling cycle of the rod from 250°C to room temperature. Three types of mechanical loads were considered, namely axial tension, small bending, and large bending conditions. This was done to predict potential mechanical failure modes, which could reduce the short term performance of the conductors. It has been shown that the magnitudes of the residual thermal stresses in the composite core are low and insufficient to create internal mechanical damage during manufacturing. As expected, the axial tension analysis indicated that under extreme axial tensile loads the ACCC rod will fail catastrophically. The most important results were obtained through the bending analysis, especially under large displacement conditions. Under these conditions the ACCC rod will develop mechanical compressive damage in its carbon fiber/epoxy section if the rods are bent around relatively small mandrels either during transportation or installation.

Comparison of the ±45° Tensile and Iosipescu Shear Tests for Woven Fabric Composite Materials

The mechanical response of a woven eight-harness satin graphite/polyimide composite has been investigated by performing ±45° tensile and Iosipescu shear tests at room temperature. Nonlinear finite element simulations of the tests have been conducted to determine internal stress distributions in the ±45° tensile and Iosipescu fabric specimens as a function of load. In the experimental part of this study, a series of tensile and Iosipescu shear tests have been performed. Acoustic emission techniques have been employed to monitor damage initiation and progression in the composite. The finite element computations have shown that the internal stress distributions in the Iosipescu and tensile fabric specimens are significantly different. In the gage sections of Iosipescu specimens, the state of stress is essentially pure shear, whereas the tensile tests generate biaxial stress conditions. It has been shown in this research that the shear strength of the composite determined from the maximum loads obtained from the Iosipescu shear tests is significantly higher than the shear strength obtained from the ±45° tensile tests. Moreover, the initiation of intralaminar damage in the tensile specimens occurs at much lower loads than in the Iosipescu specimens. It appears that the ±45° tensile test significantly underestimates the shear strength of the composite evaluated from the onset of intralaminar damage and the maximum loads.

An investigation of moisture and leakage currents in GRP composite hollow cylinders

The applicability of using flat composite plates and hollow core composite cylinders for moisture absorption testing of unidirectional glass/polymer composites used in high voltage composite (non-ceramic) insulators was examined. Two main issues were addressed in this work. First, the effect of specimen geometry (cylinders vs. plates) on moisture absorption by the composites was investigated both numerically and experimentally. Both classical Fickian and non-Fickian diffusions were considered. Subsequently, hollow core cylinders made up of ECR (low seed)-glass fibers and epoxy resin were tested for their high voltage properties under controlled moisture diffusion conditions. The specimens were exposed to warm, moist air and their high voltage properties were ascertained using a modified version of the ANSI test (standard C29.11 Section 7.4.2) for water diffusion electrical testing. It was found that the behavior of the hollow core cylinder and flat plate composite specimens subjected to moisture compared reasonably well experimentally and very well numerically. From the high voltage tests, a direct correlation was found between the amount of moisture that had been absorbed by the specimens and the amount of leakage current that was detected. It was shown that using the thin walled composite cylinders leakage currents could be predicted based on the amount of absorbed moisture in the insulator composites. The predictions can be made based on relatively short term moisture data even if the diffusion process in the composites is anomalous in nature with long times required for full saturation. After additional verifications, considering other composite systems, the hollow core cylinder testing under controlled moisture and high voltage conditions could become a screening test for selecting suitable glass/polymer composites for insulator applications.